By Paul Reynolds
World affairs correspondent, BBC News website
The argument in brief: San Francisco demonstrator awaits Olympic torch
Protests and boycotts are almost another category of Olympic sport.
In fact, it is quite rare for the Games to pass off without controversy.
"The Games are very easy targets for boycotts," Tony Bijkerk, Secretary-General of the International Society of Olympic Historians told me.
"I don't agree with them, as they hurt the athletes more than anyone else. But the Games provide a world-wide podium for protest every four years. And there is not much the Olympic movement can do about it."
When Barcelona held its successful Games in 1992, it was the first time since the Rome Games in 1960 that there were no boycotts.
Those were the heady days when the Cold War had just ended and another source of boycotts, apartheid, had also disappeared. South Africa was welcomed back that year.
Beijing's opportunity to show China's advancement into the modern world has also given demonstrators their chance to return to what is really an old Olympic tradition of protesting.
It goes back to 1908, when Irish athletes, angered at the refusal of Britain to give Ireland its independence, boycotted the Games in London.
On a smaller scale, the US team refused to dip its flag to King Edward VII in the opening ceremony.
"This flag dips to no earthly king," was the captain's comment. The US tradition of dipping its flag to nobody has continued since and will provide its own little side story when London is the host in 2012.
In 1932, there was a preview of the problems that would come four years later when, in Los Angeles, an Italian winner gave a fascist salute on the podium.
The Berlin games in 1936 (awarded to Germany before Hitler came to power) "would have to take the first prize for the most controversial", according to the Olympic Historians' Society Vice President David Wallechinsky.
The Nazis drenched the games in propaganda. There were calls for boycotts - and actual boycotts by some Jewish athletes.
But the United States did attend after Avery Brundage, President of the American Olympic Committee, overcame calls for a US boycott.
The irony is that the Games are now also remembered for the performance of the black US athlete Jesse Owens, who won four gold medals under Hitler's nose.
Incidentally, the Olympic torch relay idea was started by the Nazi organisers of the 36 Games as part of their self-glorification effort. It remains to be seen if, after this year's protests, the relay survives.
Cold War intrudes
After World War II, the Games resumed, but the Cold War began. There was a flavour of that in Helsinki in 1952, when the Soviet athletes stayed on their side of the border and came across only to compete.
In Mexico in 1968, John Carlos (right) gives black power salute...
In 1956, in Melbourne, the troubles in the Middle East made themselves felt when Egypt, Iraq and Lebanon stayed away because of the Suez invasion by Britain and France. The Cold War had an impact when the Netherlands, Spain and Switzerland refused to go because of the Soviet crushing of the Hungarian revolution.
Tokyo in 1964 saw boycotts from Indonesia and North Korea over an argument about their athletes competing in some rival games and South Africa was banned because of its racial policies.
Mexico's 1968 Games were marked by two very different protests. In the first, students demonstrated against the government about ten days before the Games and were fired on by the Mexican army. More than 200 students were killed.
Then, during the games, two black US runners, Tommy Smith and John Carlos, raised their hands in a black power salute from the podium. They were expelled on the grounds that political gestures are banned from Olympic ceremonies, but they had a huge impact.
The most disastrous games of all, in which protest moved into violence, was in Munich in 1972.
Gunmen from the Palestinian Black September group got into the Israeli compound, by climbing over an unguarded fence, and by the end eleven Israeli athletes had been murdered. The Games paused for a memorial event - and then went on.
Political influence continued in Montreal in 1976, when 26 African and Caribbean countries held a boycott because New Zealand, which had played rugby in South Africa, was allowed to compete.
Montreal started another trend in controversy - the cost of the Games. It plagued Athens and is plaguing London.
The big boycotts
The biggest boycott of them all came in 1980 when 62 countries led by the United States stayed away from Moscow following the Soviet intervention in Afghanistan the previous year.
Retaliation followed in Los Angeles in 1984. The Soviet Union led an Eastern bloc boycott.
... now Carlos demonstrates with the "human rights" torch and US flag
The Games were at a low ebb. Politics had nearly taken over.
The Seoul Games in 1988 saw something of a recovery, and even North Korea's refusal to attend, annoyed that it was not the co-host, impressed only Ethiopia and Cuba, who stayed out in sympathy.
Recovery was celebrated in a big way in Barcelona four years later and although the Atlanta Games in 1996 were marred by a bomb explosion, they were also largely free of protests.
Sydney in 2000 was judged one of the best Games ever. Athens, while hit by a large bill, went off smoothly as well.
But Beijing has shown that protests are always ready to erupt.
London can hardly be immune.